An effective transportation system plays a crucial role in the development and sustenance of a modern economy, as commerce depends on a reliable and a cost-effective method to deliver products to customers. In this context, pavements or other support surfaces for land vehicles or air vehicles during takeoff or landing phases are the backbone of the modern economy. Pavements are typically made up of a composite consisting of different sized aggregates generally excavated from earth deposits and which are designed to properly support various requirements. The primary purpose of a pavement is to transmit a load from the surface to the subgrade or underlying soil. Larger aggregates carry the load by coming into close proximity with one another, while sand or other fine aggregates fill the empty space between the larger aggregates. About 90% of all roadways and surfaces in the United States are made with asphalt, or more specifically, hot mix asphalt concrete (HMAC).
Asphalt is of particular interest to engineers because it is a strong, durable and highly waterproof cement. It is a plastic substance that imparts controllable flexibility to mixtures of mineral aggregates with which it is usually combined. It is, moreover, highly resistant to the action of acids, alkalies and salts. Although asphalt exists in a solid or semi-solid state at ordinary atmospheric temperature, it may be readily liquefied by applying heat or by dissolving it in petroleum solvents of varying volatility or by emulsifying it.
Asphalt is a natural constituent of petroleum products. The crude petroleum is refined to separate the various fractions and recover the asphalt. Similar processes occurring in nature have formed natural deposits of asphalt, some practically from extraneous matter, and some mixed with variable qualities of mineral matter. Further, asphalt can occur naturally within rocks. The rock is often referred to an asphalt impregnated rock. Hot mixed asphalt pavement consists of a combination of aggregates uniformly mixed and coated with asphalt cement. To dry the aggregates and obtain sufficient fluidity of the asphalt cement for proper mixing and workability, both must be heated prior to mixing, giving origin to the term "hot-mix".
The aggregates and asphalt are combined in an asphalt mixing plant in which they are heated, proportioned, and mixed to produce the desired paving mixture. After the plant mixing is complete, the hot-mix is transported to the paving site and spread with a paving machine in a loosely compacted layer to a uniform, smooth surface. While the paving mixture is still hot, it is further compacted by heavy self-propelled rollers to produce a smooth, well-consolidated course. The aggregates normally used are well graded, clean, cohesionless, and have high angles of internal fraction. Asphalt cement, a product of the refining of crude oil, is a reversible thermoplastic; its strength changes with temperature, as is known in the art. The viscosity of a typical paving grade of asphalt cement will be in the order of 2,500 poises at 140.degree. F. and 6,000,000 poises at 77.degree. F., and even higher at lower temperatures. This is a rather significant change when compared to the temperature change in strength of other construction materials.
Asphalt strength varies with the rate of loading. Recent research has attempted to associate the viscoelastic properties to pavement performance. The balance between durability and resistance to permanent deformation remains a constant design concern. Maximum durability is desired. However, resistance to permanent deformations cannot be overlooked. The reduced strength of asphalt cement at slow rates of loading is a desirable characteristic since it prevents the formation of regularly spaced transverse cracks in asphalt pavements. However, at this reduced strength condition, the pavement must resist excessive plastic behavior. Tensile stresses develop in all pavements as they contract during cooling. If the pavement is made with cement that has insufficient tensile strength, the tensile stress will exceed the tensile strength and cracks will occur. In pavements made with Portland cement, the tensile stresses will exceed the tensile strength when the dimension of the pavement exceeds about 15 feet. Grooves or spacers are placed at these intervals to form contraction joints that are straight and can be maintained more easily than meandering cracks.
If the strength of the asphalt cement is low enough at the rate of loading produced by contraction, the asphalt cement yields as load is applied by contraction. No significant tensile stresses build up and no transverse cracks occur. In most of the United States the climatic conditions are such that the rate of loading is slow enough that the asphalt cements normally used yield enough during contraction that transverse cracks do not develop. In the northern tier of states the climatic conditions are such that in cold weather, cooling shrinks the pavement faster than the asphalt cement can yield and thermal cracks occur.
The low strength of asphalt cement at slow rates of loading is the reason reflection cracks occur in asphalt overlays over concrete pavements. Contraction of the underlying concrete concentrates strain in the asphalt overlay directly above the joints in the concrete pavement producing tensile stress in the asphalt pavement. Since this tensile stress is applied at a slow rate of loading, the strength of the asphalt cement is very low and cracks occur in the asphalt overlay over the joints.
The design of asphalt paving mixes, as with other engineering materials designs, is largely a matter of selecting and proportioning materials to obtain the desired properties in the finished construction. The overall objective for the design of asphalt paving mixes is to determine an economical blend of binder and gradation of aggregates, within the limits of the project specifications, and an asphalt paving mixture that yields a mix having:
1. sufficient asphalt to ensure a durable pavement; PA1 2. sufficient mix stability to satisfy the demands of traffic without distortion or displacement; PA1 3. sufficient voids in the total compacted mix to allow for a slight amount of additional compaction under traffic loading without flushing, bleeding, and loss of stability, yet low enough to minimize the intrusion of harmful air and moisture. PA1 4. sufficient workability to permit efficient placement of the mix without segregation. PA1 a) Coarse aggregate, all the materials retained on the No. 8 sieve. PA1 b) Fine aggregate, all the materials passing the No. 8 sieve PA1 c) Mineral dust, that portion of the fine aggregate passing the No. 200 sieve PA1 d) Mineral filler, a finely divided mineral product, at least 70 percent of which will pass a No. 200 sieve.
Due to the importance of the proper mixture of coarse aggregates, fine aggregates and asphalt cement which in turn controls the segregation and degradation of aggregates which occurs during crushing, storage, mixing, tumbling, transportation and laydown operations, builders typically specify that the pavement contractors deploy a particular Job-Mix Formula (JMF). The job mix formula defines the actual gradation and asphalt content to be obtained in the finished construction. JMF is usually designated by the builder or contractor authority as a series of percentages associated with the number of sieves which describes the aggregate blend. As explained in U.S. Pat. No. 4,383,864, entitled "Adaptive Mix Proportioning Method For Use In Asphaltic Concrete Mixing Plants" to Trujillo, a typical Job-Mix Formula may be designated as 100% passing in a 3/4" sieve, 80-100% passing in a 1/2" sieve, 70-90% passing in a 3/8" sieve, 55-73% passing in a number 4 screen, 40-55% passing in a number 8 screen, 20-30% passing a number 30 screen, 10-18% passing a number 100 screen, and 4-10% passing a number 200 screen. Any blend of aggregates within the range designated by the Job-Mix Formula specification is generally acceptable to the builder or contracting authority, provided the proposed JMF satisfies other design criteria. The JMF thus is the combination of individual aggregates with a designed binder content that results in pavement performance. A proper aggregate gradation should have a balance of material sizes sufficient to promote particle contact and provide a controlled voids content in the compacted mixture.
In computing the JMF values, sieve sizes to be used are designated in governing specifications. Determining the percentages from weights obtained by sieve analysis. Gradations are usually expressed on the basis of total percent passing, which indicates the total percent of aggregate by weight that will pass a given size sieve. The total percent retained is just the opposite; the total percent by weight retained on a given sieve. The percent passing-retained, two successive sieve sizes or individual percent for each size group, indicates the percent retained by weight on each sieve in the sieve analysis. Certain descriptive terms used in referring to aggregate gradations are:
Conformance with the Job-Fix Formula is generally performed at a mixing plant where the asphalt cement injected into the mixing bin can be accurately controlled as a percentage by weight of the total mix. As indicated earlier, an effective amount of asphalt cement governs the amount of air voids in a compacted mixture and varies as a function of the shape, absorption characteristics, and sizes. However, as noted in Trujillo, gradation is hard to control in accordance with the Job-Mix formula at the mixing plant and at the laydown site due to degradation and segregation of the aggregates and due to the lack of adequate feeding controls for separate storage bins in the mixing plant.
U.S. Pat. No. 4,221,603, entitled "Mix Design Method For Asphalt Paving Mixtures," issued to Trujillo, shows a Mix-Design Method for determining degradation of coarse and fine aggregates to be combined to achieve a predetermined percentage of air void, volume and voids in mineral aggregates for a given quantity of asphalt cement. The method uses a volumetric value known as the Riguez Index which is derived from a compacted representative sample of fine aggregates to be used in a mixing plant. Volumes of graded aggregate composites are calculated at various gradations values below the bulking point and compared with the Riguez Index to provide the basis for graphically selecting a particular gradation wherein an aggregate mixture of the particular gradation contains the desired predetermined void volumes as when compacted. Related U.S. Pat. No. 4,357,169, entitled "Uniform Asphalt Pavement And Production Method Therefore" issued to Trujillo, shows that to control voidage of the mixing plant, respective quantities of coarse and fine aggregates injected into the mixing plant is controlled over the same single sieve side used for demarcating coarse and fine aggregates and mathematically computing the volumetric comparison. Furthermore, a stability function derived from a different combination of the crushed, fine and blend sand, and a flexibility function derived from different mix quantities of asphalt cement provide control of flexibility and stability values. This design methodology to arrive at an optimum asphaltic mixture is still a trial and error procedure. Good mixes generally result from a knowledge of aggregates, experience and luck.
During the contracting phase, contractors need an accurate forecast of costs. In addition to the expense of labor, one significant expenditure is the cost of the HMAC. However, an accurate cost projection for the HMAC is difficult, for any given aggregate blend, the effective asphalt content may vary. Coarser aggregates may require less asphalt than finer aggregates. However, coarser aggregate blends may cost more than finer aggregate blends. Thus, any cost estimate of the hot mix asphalt concrete requires an accounting of the cost of two major components, asphalt and aggregates, as well as the effect of their interactions. The variability of aggregate sizes and absorption further exacerbates the JMF analysis.
Traditionally, the process of designing Hot Mix Asphaltic Concrete mixtures is divided into three steps: the selection of an aggregate type, quality and blend, the selection of a type of asphalt binder type, and the determination of an optimum JMF (i.e., aggregates and asphalt content). Three different basic methods have been used in the design of the HMAC: a Marshall method, a Hveem method, and a Strategic Highway Research Program (SHRP) method. In the early 1900's, Mr. Francis Hveem with the California Department of Highways developed the Hveem Method of Mix Design. This process was labor intensive, requiring extensive laboratory testing and engineering analysis. The objective was to determine the optimum proportion of asphalt cement and aggregates. From the 1930's to the 1980's the Marshall Method of mix designs and its hybrids became the most preferred method of mix design. This method was successful in selecting an estimated optimum asphalt cement content. However, the resulting mix design job mix formula did not necessarily perform well in the field for all climatic and traffic loading conditions. The Marshall procedure was satisfactory in estimating optimum asphalt content, but did not correlate well with actual field performance. The most recent design method is the SHRP method. All these methods are iterative testing laboratory procedures that require extensive laboratory time and raw materials. The SHRP method of mix design further aggravates the cost and time of mix design since it requires a trial and error laboratory procedure to determine the aggregate design structure. Thus trial and error procedure has been known to take weeks to determine an acceptable aggregate skeleton.
In generating the cost estimates, the construction industry traditionally uses intuition, along with a calculator or a manual or electronic spreadsheet, to arrive at an optimum and cost effective job mix formula. Typically, the acquisition of adequate experience based on the trial and error process is quite costly and time-consuming in today's competitive environment. While spreadsheets and calculators are helpful in speeding up the estimates, they are neither easy to use nor very flexible. Present day systems typically require the user to enter various percentages, plot the results of these data inputs, and iteratively change the data until a satisfactory solution is reached. Furthermore, to the extent that these solutions provide computer-aided-optimization, the optimizing software tends to be slow and cumbersome to use. Some of these optimizations require 15 hours before a solution can be found. Furthermore, the potential least cost JMF compliance with agency mixture criteria is not known until an extensive laboratory analysis is undertaken. Often, the designer learns a potential JMF has not been successful once he or she completed an extensive laboratory study. Thus, a more efficient and easier to use system to determine the most cost effective JMF (i.e., blend of known aggregates and asphalt) and its likelihood to satisfy a mixture design criteria is needed.
Furthermore, the construction industry still uses a series of laboratory tests to determine the value of the bulk specific gravity of the laboratory molded sample G.sub.mb, which is an important parameter in the mix design. G.sub.mb is critical in determining the voids in the mixture, other volumetric properties and an optimum binder content. However, as the series of laboratory tests is iterative and repetitive, the process of running these tests is costly both in time and in materials. Also, once a designer learns that a current blend does not satisfy the criteria, he or she has to begin the process once again. Thus, a more efficient way to estimate G.sub.mb is needed. Similarly, a system for determining all the volumetric properties, including total voids in the mixture, voids in mineral aggregates (VMA), the percent of voids filled with asphalts (VFA) is also needed. The prediction of volumetric properties also permits the estimation of an optimum binder content.
Turning now to the data entry process for arriving at the job mix formula, one historical method for entering data in satisfaction of the job mix formula specification is by manual entry of data and subsequent plotting of the entered data. Based on the graphical plots, experienced engineers can blend the components by reviewing the aggregate shape. However, this data entry method is cumbersome. What is needed is a graphical method for entering the desired shape of the gradation blends and generating a list of optimized blended components automatically.